Patent Document 1 discloses a brake system having (a) a hydraulic brake configured to restrain rotation of a wheel, (b) a master cylinder, (c) an accumulator, (d) a pressure increasing mechanism utilizing hydraulic pressure of the accumulator and activatable by activation of an electric actuator, and (e) a selector valve configured to select a higher one of hydraulic pressure of the pressure increasing mechanism and hydraulic pressure of the master cylinder, and to supply the selected hydraulic pressure to a brake cylinder of the hydraulic brake.
When the electric actuator is normal, the pressure increasing mechanism is activated by the electric actuator. When the electric actuator is in failure, the pressure increasing mechanism is activated by the hydraulic pressure of the master cylinder. Further, when the working fluid of high pressure can be supplied from the accumulator, the pressure increasing mechanism can generate hydraulic pressure that is higher than the hydraulic pressure of the master cylinder. When the working fluid of the accumulator becomes low, the hydraulic pressure outputted by the pressure increasing mechanism becomes low, too.
Since the higher one of the hydraulic pressure of the pressure increasing mechanism and the hydraulic pressure of the master cylinder is selected by the selector valve so as to be supplied to the brake cylinder, the hydraulic pressure of the master cylinder is supplied to the brake cylinder when the hydraulic pressure of the accumulator is low, namely, when the hydraulic pressure outputted by the pressure increasing mechanism is low.
Patent Document 2 discloses a brake system having (a) hydraulic brakes provided for front right, front left, rear right and rear left wheels of a vehicle and configured to restrain rotations of the wheels, (b) a master cylinder, (c) a mechanical booster mechanism provided between the master cylinder and brakes cylinders of ones of the hydraulic brakes which are provided for the front right and front left wheels, (d) a high pressure source pressure of the high pressure source. In this brake system, when the high pressure source and the electromagnetic valve are normal, the hydraulic pressure of the high pressure source controlled by the electromagnetic valve is supplied to the brake cylinders provided for the front and rear wheels. When components such as the electromagnetic valve are in failure, the hydraulic pressure generated by the mechanical pressure increasing mechanism is supplied to the brake cylinders provided for the front wheel while the hydraulic pressure of the master cylinder is supplied to the brake cylinders provided for the rear wheels.